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Abstract

Magnetic Resonance - Electrical Properties Tomography, aiming at reconstructing
the electrical properties (EPs) at radio frequencies, has a continuously increasing
importance in terms of identifying the cancerous tissues and distinguishing
between ischemic and hemorrhagic stroke. The presently prominent MR-EPT
method \Convection{Reaction Equation Based MR-EPT" is still not clinically
used due to the presence of image artifacts. In this regard, the objective of
this thesis is to eliminate the low convective eld (LCF) artifact, which refers to
abrupt and point-wise image perturbations on the conductivity and permittivity
reconstructions of cr-MREPT method. Since the proposed methods involve the
use of parallel RF transmission, a multichannel transceiver array is designed
by carefully scrutinizing the original TEM resonator, proposed by J.Thomas
Vaughan in 1994. Finite Element Method (FEM) based simulations of that
structure, which includes the use of coaxial line elements (transmission lines),
are done in Comsol Multiphysics. For better practical feasibility, a microstrip
transmission line based eight{channel TEM array was designed, simulated and
constructed. Each of the eight ports of this array is matched to 50
with
re
ection coe cients as low as -40 dB at 123.2 MHz. Worst decoupling between
the ports is measured as -14 dB. With the use of quadrature excitation, clear
MRI images of experimental phantoms and highly homogeneous B+
1 maps are
obtained. Using simulations, a method to eliminate the LCF artifact from the
EP reconstructions is proposed. This method involves the use of the TEM array
in two di erent excitation con gurations. In the rst excitation, the conventional
quadrature drive is used. The second excitation, on the other hand, uses
magnitude and phase optimized RF sinusoids to produce a proper transmit eld
(B+
1 ) within the object. This intentionally adjusted (B+
1 ) eld, which comprises
high eld and low eld regions with a transition in the middle, shifts the LCF artifact towards a non-central location. Finally, data from both drive experiments
are simultaneously used to reconstruct EP's. It has been further shown that the
method can be applied to di erent patients without requiring patient-speci c B+
1
optimizations. Experimentally implementing the proposed method, another novel
algorithm to extract the phase of the transmit eld ( B+
1
) in a non-quadrature
excitation is proposed. In this algorithm, the receive phases of individual
channels, being common for quadrature and non-quadrature experiments are
found from an additional quadrature drive experiment with the use of transceive
phase assumption. Then, the transmit phase of non-quadrature drive is extracted
by subtracting the receive phases from the transceive phase distributions. Strong
consensus between the simulated and experimentally estimated transmit phases
is observed. In conclusion, the conductivity reconstructions of an experimental
phantom, with the use of the developed methods, is provided. It has been shown
that the LCF artifact is alleviated and better experimental setups are required
to fully eliminate it.